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How big is the Milky Way?



Led by Ekta Patel, a fourth-grade graduate student at the University of Arizona, the study is the first of its kind to exploit the 3D motion of satellite galaxies and compare their angular momentum with a simulated universe Close Milky Way taps the scales at 0.96 trillion times the mass of the sun. Unlike previous studies using satellite galaxy positions and velocities, this new study exploits the lack of network changes between the two. As the angular momentum of a system remains constant over time, this new method allows researchers to eliminate some of the uncertainties that threaten other approaches, paving the way for more reliable results.

"Remember a figure skater doing a pirouette," says Patel in a press release. "As she pulls in her arms, she turns faster, in other words, her speed changes, but her angular momentum remains the same for the entire duration of her action."

The study, presented to the 232rd American Astronomical Society in Denver, uses data from satellite galaxies of the Hubble Space Telescope. To obtain the 0.96 trillion solar mass estimate, the study compared the angular momenta of nine satellite galaxies with those of a simulated universe of 20,000 galaxies, just like our own. This comparison helped to map nine probability distributions – possible ranges of values ​​for the mass of our galaxy – whose ensemble resulted in an estimate of 0.96 trillion solar masses.

"Our method allows us to use simultaneous measurements of the velocities of several satellite galaxies to obtain a well-founded answer to the question of what the cold dark matter theory could predict for the mass of the halo of the Milky Way," says Author. Author Gurtina Besla in a press release.

It is not uncommon for researchers to use information from satellite galaxies to measure the mass of the Milky Way galaxy. Because we are unable to see the entire galaxy, we rely on their interactions with neighboring galaxies. The Milky Way is the proud owner of at least 50 such galaxies – called the local group – each with its own abundance of stars.

Not all of them, however, are well understood. Apart from the Magellanic Clouds, which are clearly visible to the naked eye, all other satellite galaxies are extremely difficult to detect even with telescopes, making it difficult to determine if they even exist. The luminosity of a satellite galaxy is often used to estimate its mass, but the orbital motions do not always match the results of the previous method. To explain this imbalance between what we can discover and the invisible mass in our universe, researchers turn to the theory of cold dark matter.

The theory is that dark matter consists of heavy, slow-moving particles that make up about 85% of the universe's matter. This type of dark matter interacts weakly with visible matter to form small lumps, which are later contracted into larger bodies.


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